1. Field of the Invention
The present invention relates to a planar shape characteristic measuring apparatus and a planar shape characteristic measuring method which measure, for example, characteristic values relating to the shape of a measured surface of a disk such as an optical disk.
2. Description of the Related Art
As shown in FIG. 7, a disk-shaped recording medium, that is, an optical disk 101, comprises an optical disk substrate 103 made of an optically transparent plastic provided on one surface with a data recording surface 104 and provided with a data recording area 102 within a predetermined area in the data recording surface 104.
Also, there are known an optical disk where the data recording area 102 in the data recording surface 104 is provided with, for example, as shown in FIG. 8A, a continuous groove 105 and a land 106 adjacent thereto provided spirally for every track by a predetermined track pitch 109 (1 to 2 xcexcm) on the data recording surface 104 of one side of the disk substrate 103 and an optical disk where, as shown in FIG. 8B, the data recording surface 104 is provided with a series of pits 108 spirally for every track by a predetermined track pitch.
For example, in most data recordable optical disks of the phase-changing type or opto-magnetic type, a phase changing film or a magnetic film, a light reflecting layer, and a protective film layer (all not shown) are formed in that order on the data recording surface 104 on which the grooves 105 shown in FIG. 8A is provided. One of the grooves 105 and the lands 106 on the data recording surface 104 is used as a recording area, while the other is used as a light reflecting area for tracking.
Also, in most write-once (read-only) type optical disks, a light reflecting layer and a protective film layer (both not shown) are formed in that order on the data recording surface 104 on which the series of the pits 108 shown in FIG. 8B is provided. The series of the pits 108 on the data recording surface 104 is used as both a recording area and a diffraction grating for tracking.
In an optical disk having the above configuration, a laser beam condensed by an object lens (not shown) mounted on an optical pickup is fired from the non-data surface 107 at the side opposite to the data recording surface 104 of the optical disk substrate 103 while rotating the optical disk.
In a data recordable optical disk, information is optically recorded in a recording layer on the land 106 by the beam or the information optically recorded on the recording layer is read by the reflected light beam. Further, for example the light beam reflected from the groove 105 is detected for tracking so that the laser beam for recording or reproduction is always focused on a predetermined track.
In a write-once type (read-only) optical disk, information is read and tracking performed by detecting the reflected and diffracted beam from the surface 104 provided with the series of pits 108 due to the beam from the non-data surface 107 to the optical pickup.
On the other hand, in the high density optical disks being developed in recent years, as shown in FIG. 9, ones are known having an optical disk substrate 103 provided with grooves 105 and formed with a light reflecting surface 104 comprising a light reflecting layer and a phase change film or a magnetic film and a transparent layer 111 which has a constant thickness of about 0.1 mm in that order.
Similarly, in a write-once type (read only) optical disk, there is known an optical disk formed with a light reflecting surface 104 comprising a light reflecting layer and a transparent layer 111 having a constant thickness about 0.1 mm in that order.
In the case of an optical disk formed with a film in this way, an optical pickup (not shown) is arranged at the side of the transparent layer 111 formed on the surface 104 of the optical disk substrate 103 and fires a laser beam while the optical disk is being rotated.
Summarizing the problems to be solved by the present invention, the optical disk substrate 103 is generally formed by injection molding of plastic. In an optical disk substrate 103 formed by this method, it is known that warping occurs along with heat distortion at the time of molding and changes in the environment such as the air temperature or humidity. Further, undulation occurs at the surface of the disk due to warping of the molds at the time of molding.
If rotating the optical disk substrate 103 in a state with warping or unevenness of undulation, up-down vibration occurs at the surface of the optical disk substrate 103 leading to the focal position of a data reading lens deviating from the data recording surface of the disk (defocus state) or the data recording surface becoming tilted from a focal surface of the data reading lens (skew state).
At this time, a spot condensed by the optical pickup ends up being influenced by the aberration. The magnitude of the aberration depends on the numerical aperture (NA) of a pickup lens. The aberration caused by the defocus is proportional to the second power of the numerical aperture (NA), while the aberration caused by the skew is proportional to the third power of the numerical aperture (NA). That is, due to the larger numerical aperture (NA), the allowable defocus and skew become smaller.
On the other hand, the smaller the thickness from the surface of the disk to the data recording surface 104, the smaller the aberration due to the defocus and the skew. For this reason, along with the recent higher density of optical disks, the numerical aperture (NA) has become higher and the thickness of the disk has become smaller.
For example, the high density optical disk such as shown in FIG. 9 is structured with a thin transparent layer 111 of a thickness of about 0.1 mm placed on the data recording surface 104 on which the grooves and the lands or the series of pits of the disk substrate 103 are formed.
Thus, to decrease or avoid the influence of the aberration caused by the increase of the numerical aperture (NA), an optically thin disks are being developed. Recently, a high density disk having a short wavelength (xcexxe2x89xa6430) and a large numerical aperture (NAxe2x89xa70.76) has been proposed.
Because the depth of focus becomes shallower along with the higher NA as stated above, stricter values are now demanded for the surface vibration permitted to the optical disk substrate 103, that is, the magnitude of the unevenness of the surface of the optical disk substrate 103.
For this reason, it is necessary to accurately measure the characteristics relating to the shape of the surface of the optical disk substrate 103. For example, it is demanded to precisely and efficiently measure the unevenness of the surface of the optical disk substrate 103, the amount of focus servo error making it impossible to track the unevenness of the surface of the optical disk substrate 103 generated at the time of focus servo control or the so-called xe2x80x9ctangential skewxe2x80x9d, and other planar shape characteristics.
An object of the present invention is to provide a planar shape characteristic measuring apparatus and a planar shape characteristic measuring method which can precisely and efficiently measure characteristics relating to the shape of a measured surface of a disk such as a data recording surface of an optical disk.
According to a first aspect of the present invention, there is provided a planar shape characteristic measuring apparatus comprising a speed detecting means for detecting a perpendicular direction speed of a measured surface of a rotating disk and a tilt angle calculating means for calculating a tilt angle in the rotational direction of said measured surface relative to a reference surface at each measuring position based on the linear speed at each measuring position detected by said speed detecting means and the detected perpendicular direction speed.
Preferably, the planar shape characteristic measuring apparatus further comprises a digital filtering means for removing a noise component contained in speed data detected by said speed detecting means and a filter coefficient calculating means for calculating a filter coefficient in accordance with a sampling rate of said speed detecting means so that a filter characteristic of said digital filtering means becomes a predetermined filter coefficient.
Preferably, said tilt angle calculating means uses a value of said perpendicular direction speed divided by said linear speed as a tangent of said tilt angle and calculates said tilt angle by an inverse function of the tangent.
Alternatively, the planar shape characteristic measuring apparatus further comprises a rotating means for rotating said disk; a moving means for relatively moving said speed detecting means in the radial direction of said disk; an input means for inputting a radius of said disk at a measuring position detected by said speed detecting means and a target linear speed at said measuring position; and a control means for calculating a speed for rotating said disk based on the input radius and target linear speed, rotating said disk at the calculated speed, and outputting a control command for moving said speed detecting means to the measuring position of said input radius to said rotating means and moving means.
Preferably, said speed detecting means detects said perpendicular direction speed by a non-contact means at a measured surface of said disk.
Preferably, said speed detecting means comprises a laser Doppler speed meter.
Preferably, said disk comprises a recording medium enabling at least one of optically recording and reproduction of information; and said measured surface comprises a data recording surface of said recording medium.
Alternatively, the planar shape characteristic measuring apparatus further comprises a displacement calculating means for calculating a perpendicular direction displacement of said measured surface based on the detected perpendicular direction speed; an acceleration calculating means for calculating a perpendicular direction acceleration of a measured surface of said disk at each detecting position based on the detected perpendicular direction speed; and a focus servo error calculating means for calculating an amount of focus servo error which does not able tracking of unevenness of said measured surface predicted to occur in a servo system for making a predetermined object track a target position in a perpendicular direction from a measured surface of said rotating disk based on the detected perpendicular direction speed and a gain characteristic of said servo system.
More preferably, said object is an optical pickup for performing at least one of recording of information onto a data recording surface of said recording medium and reproduction of information from said data recording surface or a lens mounted on said optical pickup; and said servo system comprises a focus servo system for making the focus of the optical pickup or the lens mounted on said optical pickup track a target position in a direction perpendicular to said data recording surface.
Preferably, said focus servo error calculating means comprises a band-pass filter for removing high and low frequency noise components contained in detected perpendicular direction speed data and an integrator for integrating by time series the output from said band-pass filter.
According to a second aspect of the present invention, there is provided a planar shape characteristic measuring method comprising a speed detecting step for detecting a perpendicular direction speed of a measured surface of a rotating disk and a tilt angle calculating step for calculating a tilt angle in the rotational direction of said measured surface with respect to a reference surface at each measuring position based on a linear speed of said measured surface at each measuring position and the detected perpendicular direction speed.
Preferably, the method further comprises a filtering step for removing a noise component contained in speed data detected in said speed detecting step by a digital filter and a filter coefficient calculating step for calculating a filter coefficient in accordance with a sampling rate of said speed detecting step so that a filter characteristic of said digital filter becomes a predetermined filter characteristic.
Preferably, said tilt angle calculating step uses a value of said perpendicular direction speed divided by said linear speed as a tangent of said tilt angle and calculates said tilt angle by an inverse function of the tangent.
Preferably, said speed detecting step comprises a step for inputting a radius of said disk at a measuring position and a target linear speed at said measuring position, a step for calculating a speed for rotating said disk based on the input radius and target linear speed and for rotating said disk at the calculated speed, and a step for moving a predetermined speed detecting means which detects said perpendicular direction speed to the measuring position of the input radius.
Alternatively, said speed detecting step detects said perpendicular direction speed by a noncontact means at a measured surface of said disk.
More preferably, said speed detecting step detects said perpendicular direction speed by a laser Doppler speed meter.
Preferably, a recording medium which enables at least one of optical recording and reproduction of information is used for said disk; and said measured surface comprises a data recording surface of said recording medium.
Alternatively, the method further comprises a displacement calculating step for calculating a perpendicular direction displacement of said measured surface based on the detected perpendicular direction speed; an acceleration calculating step for calculating a perpendicular direction acceleration of a measured surface of said disk at each detecting position based on the detected perpendicular direction speed; and a focus servo error calculating step for calculating an amount of focus servo error which does not able tracking of unevenness of said measured surface predicted to occur in a servo system for making a predetermined object track a target position in a perpendicular direction from a measured surface of said rotating disk based on the detected perpendicular direction speed and a gain characteristic of said servo system.
More preferably, said object is an optical pickup for performing at least one of recording of information onto a data recording surface of said recording medium and reproduction of information from said data recording surface or a lens mounted on said optical pickup; and said servo system comprises a focus servo system for making the focus of the optical pickup or the lens mounted on said optical pickup track a target position in a direction perpendicular to said data recording surface.
More preferably, said focus servo error calculating step removes high and low frequency noise components contained in the detected perpendicular direction speed data by a band-pass filter and calculates said amount of focus servo error by integrating by time series the output from this band-pass filter.
In the present invention, the perpendicular direction speed of a rotating disk is detected and the tilt angle in the rotational direction at the measuring position, the so-called tangential skew, is calculated from the detected perpendicular direction speed and the linear speed at a measuring position.
The linear speed at a measuring position can be specified from a radius of the measuring position and a speed of the disk. Because the perpendicular direction speed of the disk is a value detected directly by a laser Doppler speed meter for example, the precision of calculation of the tangential skew can be increased.